Bøger i Geophysik / Geophysics serien

Sect. 14. 297 for such slow variations the effect described under 2. above will be by far the most predominant. In fact, the equipment provides a record of the time derivative of the element in question. 6) Records of the time derivatives of magnetic elements can, however, be obtained in a more direct way by recording directly the electromotive force (e. m. f. ) induced in suitably disposed coils by the variations of the geomagnetic field 10, 11. When a coil with the total winding area F is placed with its axis in the direction of a geomagnetic force component the variation of magnetic induction B with time will produce an electromotive force 1 dB 1 dH e=- coveu, uuFdT=- 'oV, uQf1oFdT (i). )) . !. . =_to-13 d(B/y) (1). )a) v em! d(tjs) in the coil. The e. mJ. may be amplified and recorded by means of an oscillograph or on a tape recorder. but more often the coil is used in connection with an ordinary galvanometer and the scale value of, for instance the dl1/dt record, will then - in the same way as the scale value of the magnet variometer treated in Subsect. IX - be dependent on the damping factor IX and on TO/T, where To and T are the oscillation periods for the undamped galvanometer and for the magnetic pulsation respectively. The amplitude recorded will be proportional to the amplification factor I, obtained from Eq. (1).

The lower diagrams seem to indicate that the number of auroras seen in the northermost region (58 Degrees -60 Degrees) varies relatively little throughout the year, but at lower latitudes during equinoctial periods the belt in which overhead auroras are seen becomes, on the average, broad.

83 input to the ions is balanced by a cooling to atomic oxygen, the major neutral constituent, and measurements of the difference between ion and neutral tem perature permit the determination of atomic oxygen concentrations.